This invention concerns electricity distribution networks in general and in particular, but not exclusively, medium or low voltage electricity distribution networks in urban areas. Specifically, the invention deals with the problems relating to stray voltage in low voltage electricity distribution systems via interconnection or junction lines and boxes located below the road surface in urban areas.
Electricity is distributed over long distances by raising the voltage to very high values, in the order of hundreds of kV, to reduce the losses due to the Joule effect. The electricity is then transformed in transforming stations from high voltage (HV) to medium voltage (MV), in the order of dozens of kV. These stations are located near the utilities. Capillary distribution, for example to the buildings of an urban area, along the roads and if necessary also to the public lighting systems of the same urban area is performed at low voltage (LV). For said purpose, a plurality of transformers from medium to low voltage (MV/LV) connect the medium voltage lines to the low voltage distribution network, where the electricity is distributed at typical voltages of 220 V or 120 V or other values established by the standards of each country.
The low voltage electricity distribution networks are often laid underground, i.e. arranged below the road surface or the pavement. They are provided with numerous interconnection boxes, which can vary in size. Ducts, inside which the cables from other interconnection boxes or from the MV/LV transformers run, lead into the interconnection boxes. The ducts have the function of protecting the underground cables from damage caused by atmospheric agents, human activity or the aggression of rodents. Inside the interconnection or junction boxes, different interconnection lines are connected to one another via terminals or other suitable devices. Normally, each interconnection line contains a plurality of cables for each phase. When the distribution system is a three-phase system, each interconnection line will contain a plurality of cables for each phase in addition to the neutral or several cables of the neutral. The use of a plurality of cables for each phase is necessary in view of the high current intensity that has to be transmitted.
This means that inside each junction or interconnection box there are multiple connections between different cables. These connections are insulated to avoid dispersion. However, due also to the particular arrangement of the interconnection or junction boxes, the insulation is subject to wear or can become less efficient. In this case the interconnection box can come into electrical contact with a phase of the distribution network. To prevent this from causing accidents, which can be fatal, following accidental contact with the live box, these boxes are grounded to keep them at zero voltage with respect to the ground.
Grounding of these boxes can be faulty, however, in particular due to the formation over time of layers of oxide with high electrical resistance. The consequence of this is that an accidental electrical contact between the interconnection box and a phase of the distribution network can energize the box or part of it (for example the cover). Simultaneous accidental contact between the box and the ground for example by a pedestrian can have fatal consequences.
Currently the junction or interconnection boxes of the electricity distribution network are periodically inspected to check for correct grounding. However, in large urban areas there may be tens of thousands or hundreds of thousands of these boxes. Periodical checking of them means that a very long time, even years, elapses between one check and the next one, with consequent total unreliability and risk of electrocution.